Well...Getting inspiration from the actual act is definitely one way to do it.
In Hentai, when your editor is a woman, more often than not this happens.
I'm actually more surprised it didn't work out more than anything by that ending. Normally everything goes well after sex...I guess there's always the next time?

Okay, writer in me's gonna speak up: Motivation? Motivation's for amateurs! The rest of use show up, and get to work! (To paraphrase Chuck Close). (Feel free to point me at this post if I use this excuse in your hearing)

I do have a question— given that what you just did was straight out of hetai WHY DIDN'T YOU JUST DRAW THAT YOU SILLY SKIVVY SENSEI WANNABES!